Two southwestern desert landscape trees, Acacia smallii L. (sweet acacia) and Cercidium floridum Benth. ex A. Gray (blue palo verde), were grown outdoors in full sun during Summer 1997 in 19-liter (#5) containers placed either pot-in-pot (PIP) below ground or unshielded in above-ground containers (AGC). Soil moisture sensors wired to electronic solenoid valves regulated occurrence of six cyclic micro-irrigation pulses per day (0600, 0900, 1200, 1500, 1800, and 2100 HR) such that container substrate moisture tensions were continuously maintained between −0.005 to −0.01 MPa (90% of water holding capacity) in both PIP and AGC. Mean maximum recorded root-zone temperatures in PIP containers were 19C (34F) lower than for AGC. Micro-irrigation volumes were 40% less for trees grown PIP compared with those in AGC. Growth of sweet acacia was enhanced by PIP placement while in containers and one year after transplanting trees into field plots in 1998. Only caliper growth of blue palo verde was increased by PIP placement while in containers, but had no effect on blue palo verde growth one year after transplanting into field plots. The critical killing temperature (TM) for root tissues of sweet acacia and blue palo verde were 45.3 ± 1.8C (113.5 ± 3.2F) and 49.4 ± 0.8C (120.9 ± 1.4F), respectively, indicating differences in root membrane thermostability. Based on our data, we suggest that sweet acacia trees benefitted from PIP placement more than blue palo verde trees because root-zone temperatures in PIP containers were lower than for AGC in central Arizona, and sweet acacia roots were more susceptible to injury by supraoptimal root zone temperatures.
This research was funded, in part, by a grant from The Horticultural Research Institute, Inc., 1250 I Street, N.W., Suite 500, Washington, DC 20005.
3Graduate Research Assistant.
4Research Scientist, address: DesiertoVerde Nursery, 910 NorthAlma School Rd., Scottsdale, AZ 85284.